In recent years, hydrofluorocarbon(s) (HFC) that have ozone depletion potential of zero have been used as a refrigerant for a refrigerator and the like. However, since the HFC still have high global warming potential (GWP), a refrigerant that has lower GWP has been desired. At present, fluoropropene refrigerants such as 2,3,3,3-tetrafluoro-1-propene (hereinafter referred to as “HFO-1234yf”) and 1,3,3,3-tetrafluoro-1-propene (hereinafter referred to as “HFO-1234ze”); a mixed refrigerant of fluoropropene and HFC; and the like have been considered to be candidates for such a refrigerant (see Patent Document 1 and Non-patent Document 1).
A refrigerator is normally configured so that a refrigerant oil circulates through the refrigerant circulation cycle together with a refrigerant that lubricates a refrigerant compressor. Therefore, the refrigerant oil is required to have miscibility with the refrigerant. Since the refrigerant oil is used to lubricate the operational parts of the refrigerator, it is important for the refrigerant oil to also have lubricity. When the refrigerant oil undergoes phase separation, the refrigerant oil discharged from the refrigerant compressor may easily remain within the cycle. As a result, the amount of the refrigerant oil in the refrigerant compressor may decrease and a lubrication failure may occur, and/or an expansion mechanism (e.g., capillary) may be clogged. It is important to retain the oil film inside the compressor with high operating temperature in order to implement lubricity inside the refrigerator, and the viscosity of the refrigerant oil is an important factor for retaining the oil film. If the viscosity of the refrigerant oil is low, the thickness of the oil film may decrease, and a lubrication failure may occur. If the viscosity of the refrigerant oil is high, the heat exchange efficiency may decrease (see Patent Documents 2 and 3).
An industrial lubricant such as a refrigerant oil is normally required to have excellent low-temperature fluidity for use in a low-temperature environment (e.g., winter or a cold district) and also to have improved stability. Examples of the stability include thermal stability, oxidation stability, oxidation-hydrolysis stability, and the like. An apparatus that utilizes the lubricant is required to have improved durability (e.g., wear resistance and fatigue resistance), improved energy-saving capability, and the like.
Patent Document 4 discloses a refrigerant oil that consists of a tetraester of pentaerythritol and 2-ethylhexanoic acid. However, Patent Document 4 does not disclose or suggest the miscibility of the ester with a refrigerant that comprises fluoropropene. Non-patent Document 2 discloses the pour point of a tetraester of pentaerythritol and 2-ethylhexanoic acid. However, the low-temperature fluidity required for an industrial lubricant (e.g., refrigerant oil) and the like is unsatisfactory.
Patent Document 5 discloses a lubricant base stock for an automotive/aeronautic engine/turbine and the like that comprises a tetraester of 2-propylheptanoic acid and pentaerythritol. Patent Document 5 does not disclose or suggest the miscibility of the ester with a refrigerant that comprises fluoropropene.
Patent Document 1 discloses a refrigerant oil that comprises an ester of pentaerythritol and a mixed fatty acid that consists of 2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acid (mixing ratio(molar ratio): 50/50), and discloses the miscibility of the ester with HFO-1234yf at 0° C. Patent Document 6 discloses the pour point of the ester. However, the lubricity and the like of the ester are unsatisfactory, and the performance required for an industrial lubricant (e.g., refrigerant oil) and the like is not achieved in a well-balanced manner.